Corrosion resistant steels known as stainless steels have long been known and are presently available with a variety of properties. Austenitic stainless steels, which are those consisting substantially of a single austenite phase, possess the best properties of corrosion resistance and good mechanical properties, particularly at high temperature.
Austenitic stainless steels in the past have been steels in which chromium and nickel are the principal alloying agents. However, nickel is not an abundant metal and the increased demand for it has increased its price and made its supply uncertain, particularly in critical times. Substitutes for nickel in the chromium-nickel austenitic stainless steels have long been sought. Recently the combined use of manganese and nitrogen along with chromium in carefully limited ratios has produced an austenitic stainless steel. In that steel, at least 0.85% nitrogen is required to produce the austenitic structure but large amounts of nitrogen cause the steel ingot to be porous. Accordingly, it has been found that the nitrogen content of such a steel must be limited to less than 3%. Throughout the specification and claims all references to percent in composition shall be percent by weight of the total composition.
The manganese in the above-noted steel must be present in amounts of from 15-45%. Manganese is necessary to increase the solubility of nitrogen in steel, in addition to itself contributing to the austenitic structure.
Although the chromium-manganese-nitrogen stainless steel is an excellant austenitic stainless steel, it has been found to be subject to attack by sulfuric acid to a significant extent and accordingly its use is limited to those environments where sulfuric acid is not present.